The present invention relates to an apparatus for producing water containing dissolved ozone. More particularly, the present invention relates to an apparatus for removing substances promoting decomposition of ozone which are contained in ultrapure water in very small amounts and providing water containing dissolved ozone which shows little decrease in the concentration of ozone and has a great residual fraction of ozone dissolved in water after transportation for a long distance.
For achieving excellent quality and high yields of products of electronic materials such as silicon substrates for semiconductors, glass substrates for liquid crystals and quartz substrates for photomasks, it is important that foreign substances are removed from the surface of the electronic materials. Wet washing is widely conducted for this purpose. Application of a washing fluid having a strong oxidizing ability is effective for removing organic and metal contaminants. Washing at a high temperature with a mixed solution of sulfuric acid and hydrogen peroxide (the SMP washing liquid) or a mixed solution of hydrochloric acid, hydrogen peroxide and ultrapure water (the SC2 washing liquid) has heretofore been conducted.
It is required recently that the washing be conducted in accordance with a simplified process using less materials at the room temperature. Therefore, water containing dissolved ozone which exhibits very strong oxidizing ability and effectively removes organic and metal contaminants on the surface of electronic materials even when the concentration of ozone is several mg/liter is being used for wet washing. Water containing dissolved ozone is characterized in that the water provides ordinary high purity water when the dissolved ozone is decomposed. However, since the dissolved ozone is decomposed with time and converted into oxygen, it has been considered to be difficult that the concentration of ozone is maintained well and water containing dissolved ozone is transported for a long distance through a piping. Therefore, water containing dissolved ozone is prepared in the vicinity of a cleaning apparatus and used immediately after being prepared.
It was found by the present inventors that the decrease in the concentration of ozone was suppressed and water containing dissolved ozone could be transported for a long distance when ultrapure water was transported while a gas containing ozone was mixed with the ultrapure water in a piping for the transportation. An apparatus for supplying water containing dissolved ozone shown in FIG. 1 is proposed in Japanese Patent Application Laid-Open No. Heisei 11-138181. Oxygen gas from a container of oxygen gas 1 is mixed with a very small amount of nitrogen gas from a container of nitrogen gas 2. The resultant mixed gas is introduced into an ozone generator 3 of the silent discharge type and a mixed gas of ozone and oxygen gas is produced. The produced mixed gas is introduced into ultrapure water in an apparatus for dissolving ozone 4 using an ejector or a pump. The ultrapure water has been prepared using an ion exchange apparatus, a membrane apparatus, an apparatus for irradiating with ultraviolet light and other necessary apparatuses. The mixed gas of ozone and oxygen gas is mixed with the ultrapure water and a product in the condition of a gas-liquid mixture is formed. Ozone is dissolved into water in the product and water containing dissolved ozone is formed. The product flows through a piping for transporting a gas-liquid mixed fluid 5 while remaining in the condition of a gas-liquid mixture. Although ozone dissolved in water is converted into oxygen gas by spontaneous decomposition, the amount of ozone decreased due to the spontaneous decomposition is supplemented with ozone dissolved into the water phase from the gas phase and the concentration of ozone can be kept approximately constant. The water containing dissolved ozone is taken out of branched pipes 6, subjected to gas-liquid separation and consumed at points of use 7. The water containing dissolved ozone which is present in an excessive amount and not taken out of the branched pipes is introduced into a gas-liquid separator 8 and separated into the gas phase and the water phase. Ozone in the gas phase is decomposed in an apparatus for decomposing ozone 9 and ozone in the water phase is decomposed in an apparatus for decomposing ozone 10. After the de composition of ozone has been completed, the gas phase is released to the atmosphere as a waste gas and the water phase is recovered as waste water and reused after necessary treatments.
When the above apparatus for supplying water containing dissolved ozone is used, change in the concentration of ozone during transportation is small even when the water containing dissolved ozone is transported for a long distance and water containing dissolved ozone in an approximately constant concentration can be supplied to points of use. However, ozone is required in an excessive amount in this apparatus since the amount of ozone in the water containing dissolved ozone decreased due to the spontaneous decomposition is supplemented with ozone dissolved into the water phase from the gas phase. Therefore, water containing dissolved ozone in which decomposition of ozone is suppressed and which shows little decrease in the concentration of ozone and has a great residual fraction of ozone dissolved in water after transportation for a long distance is required.
The present invention has a object of providing an apparatus for producing water containing dissolved ozone which can provide water containing dissolved ozone which shows little decrease in the concentration of ozone and has a great residual fraction of ozone dissolved in water after transportation for a long distance.
As the result of intensive studies by the present inventors to achieve the above object, it was found that the decrease in the concentration of ozone took place due to the presence of very small amounts of substances promoting decomposition of ozone in ultrapure water used as the material water for water containing dissolved ozone, the substances in very small amounts were effectively removed by bringing the substances into contact with an oxidation-reduction catalyst and the substances in very small amounts were formed by excessive irradiation with ultraviolet light in the process for producing ultrapure water. The present invention has been completed based on the knowledge.
The essence of the present invention is to remove very small amounts of substances promoting decomposition of ozone from ultrapure water supplied to an apparatus for dissolving ozone by bringing at least one of the ultrapure water supplied to the apparatus for dissolving ozone and the water used in the process of producing ultrapure water into contact with an oxidation-reduction catalyst.
The present invention provides:
(1) An apparatus for producing water containing dissolved ozone which comprises (A) a piping for supplying ultrapure water through which ultrapure water is supplied, (B) a catalytic reaction portion which is connected with the piping for supplying ultrapure water and in which the ultrapure water is brought into contact with an oxidation-reduction catalyst, (C) a filtration apparatus by which the ultrapure water treated in the catalytic reaction portion is filtered and (D) an apparatus for dissolving ozone in which ozone is dissolved into the ultrapure water discharged from the filtration apparatus; and
(2) An apparatus for producing water containing dissolved ozone which comprises an apparatus for producing ultrapure water which is equipped with an apparatus for irradiating with ultraviolet light, an apparatus for dissolving ozone in which ozone is dissolved into the ultrapure water produced in the apparatus for producing ultrapure water and a catalytic reaction portion which is packed with an oxidation-reduction catalyst and disposed between the apparatus for irradiating with ultraviolet light and the apparatus for dissolving ozone.
The phenomena as the basis of the present invention will be described in the following. When water containing dissolved ozone is produced by dissolving ozone into ultrapure water produced by an apparatus for producing ultrapure water equipped with an apparatus for irradiating with ultraviolet light, it occurs occasionally that ozone dissolved in water shows a great rate of spontaneous decomposition and the concentration of ozone rapidly decreases. As the result of studies on the cause of this phenomenon, it was found that the rate of spontaneous decomposition of ozone increases when the amount of ultraviolet light used for the irradiation is more than necessary. When ultraviolet light is used in an amount more than necessary for decomposing organic carbons (TOC), ultraviolet light works on water molecules in addition to TOC to be decomposed and OH radical and H radical are formed. Although these radicals form the water molecule by recombination, it is estimated that radical substances continue to be present for a while as intermediates for the recombination and promote the decomposition of ozone.
In the production of ultrapure water, target values are set as the quality of ultrapure water to be produced and, in general, TOC is included as one of the target values. When fluctuation in the concentration of organic substance residual in the primary pure water is considered, it is advantageous for the process control that the amount of ultraviolet light used for the irradiation is set at an excessive amount so that the target value of TOC can be achieved even when the concentration of TOC has the maximum value in the estimated range of fluctuation. Irradiation with ultraviolet light in an excessive amount may be prevented by adjusting the amount of ultraviolet light used for the irradiation in accordance with the fluctuation in TOC in the primary pure water. However, there is the possibility that the facilities and the process control become complicated. When water obtained after irradiation with ultraviolet light is brought into contact with an oxidation-reduction catalyst using the apparatus of the present invention, the decomposition of ozone dissolved in water can be suppressed. Therefore, the irradiation with ultraviolet light can be conducted always in an excessive amount so that TOC in the primary pure water can be sufficiently decomposed and stability of water containing dissolved ozone can be improved without major change in the existing apparatus for producing water containing dissolved ozone. The mechanism of the improvement in stability of water containing dissolved ozone by the apparatus of the present invention is not fully elucidated. It is considered that the OH radical and the H radical generated in the water by irradiation with ultraviolet light in an excess amount and radical substances present as intermediates in recombination of these radicals to form the water molecule are rapidly converted into the water molecule by the contact with the oxidation-reduction catalyst and disappear.